Microwave circuit boards consist of thin sheets of dielectric material sandwiched between two layers of copper or other metal. The thickness of the dielectric sheets is typically in the range of 0.01-0.3 inches. The boards are used for a variety of applications involving the transmission of microwave signals. The copper layers on one or both sides of the board may be etched into intricate shapes as required by a particular application.
Unlike more common low frequency circuit boards, microwave circuit boards must employ a low loss dielectric material to prevent unwanted heating and dissipation of energy. Further, and of particular importance to the present invention, the speed at which microwave energy propagates through the boards depends on the dielectric constant of the dielectric sheets. Testing and control of the dielectric constant of such sheets is, therefore, critical to reliable circuit design. The use of routine testing methods is precluded, however, by the fact that the sheets are sandwiched between copper layers that block the transmission of microwave energy.
Prior methods for testing the dielectric constant of microwave circuit boards include low frequency capacitance measurements, pull-tab methods and ultrasonic methods. Capacitance measurements simply yield board-wide averages, and provide no information concerning variations within a board. A localized anomaly in dielectric constant will therefore go undetected using a capacitance technique. In the pull-tab method, copper is removed from a small section of the board, and the underlying substrate is tested using a dipole-resonant frequency technique. Unfortunately, this method destroys the board under test, and also fails to sample the entire board. Ultrasonic sounding is capable of finding local delaminations and of determining the relationship between dielectric constant and density. However, it has not yet been perfected to the point where it can reliably be used for the direct measurement of dielectric constant.